Passive infrared intrusion detection systems are known for sensing the presence of an intruder in a protected space and for providing an output signal representative of intruder detection. Examples of passive infrared intrusion detection systems are shown in U.S. Pat. Nos. 3,036,219; 3,524,180; 3,631,434; 3,703,718; 3,886,360; and 4,375,034. It is an object of the present invention to provide a system and a mirror assembly therefor especially suited to ceiling mounting to produce several selectively steerable curtains through which an intruder must pass when moving through the area of a particular protected region.